climatic impact of arctic ocean methane hydrate dissociation ......climatic impact of arctic ocean...

Climatic impact of Arctic Ocean methane hydrate

dissociation in the 21st-century

Sunil Vadakkepuliyambatta1*, Ragnhild B Skeie2, Gunnar Myhre2, Stig B Dalsøren2, Anna

Silyakova1, Norbert Schmidbauer3, Cathrine Lund Myhre3, Jürgen Mienert1

1CAGE-Center for Arctic Gas Hydrate, Environment, and Climate, Department of Geosciences, UiT-The Arctic University of Norway, 9037

Tromsø, Norway.

2CICERO-Center for International Climate and Environmental Research – Oslo, PB. 1129 Blindern, 0318 Oslo, Norway.

3NILU - Norwegian Institute for Air Research, Instituttveien 18, 2027 Kjeller, Norway.

Abstract 1

Greenhouse gas methane trapped in sub-seafloor gas hydrates may play an important role in a 2

potential climate feedback system. The impact of future Arctic Ocean warming on the hydrate 3

stability and its contribution to atmospheric methane concentrations remains an important and 4

unanswered question. Here, we estimate the climate impact of released methane from oceanic 5

gas hydrates in the Arctic to the atmosphere towards the end of the 21st century, integrating 6

hydrate stability and atmospheric modeling. Based on future climate models, we estimate that 7

increasing ocean temperatures over the next 100 years could release up to 17 ± 6 Gt C into the 8

Arctic Ocean. However, the released methane has a limited or minor impact on the global 9

mean surface temperature, contributing only 0.1 % of the projected anthropogenic influenced 10

warming over the 21st century. 11

1. Introduction 12

Methane is a greenhouse gas, which has a global warming potential ~28 times greater than 13

CO2 over 100 years (Myhre et al., 2013). The gas contributes significantly to the recent 14

increase in global temperature and increasing atmospheric greenhouse gas levels (Hartmann et 15

al., 2013;Saunois et al., 2016). A large amount of methane in the ocean is trapped in 16

sediments in the form of gas hydrates, an ice-like crystalline substance made of water and gas 17

(Sloan and Koh, 2008), which occurs in the pore space of sediments in continental margins all 18

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Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2017-110Manuscript under review for journal Earth Syst. Dynam.Discussion started: 18 December 2017c© Author(s) 2017. CC BY 4.0 License.

over the world (Collett et al., 2009). Gas hydrates are generally stable under high-pressure and 19

low-temperature but are extremely sensitive to slight variations in these conditions. The 20

release of methane from dissociating hydrates in response to a warming ocean has been 21

suggested to have major implications for past and rapid warming events (Dickens et al., 1997) 22

resulting in a positive climate feedback (Berbesi et al., 2014;Kroeger and Funnell, 2012). The 23

potential impact of hydrates on global climate and its consideration as an unconventional 24

energy resource lead to numerous studies trying to estimate the amount of methane trapped in 25

hydrates beneath the ocean floor (Archer et al., 2009;Buffett and Archer, 2004;Burwicz et al., 26

2011;Dobrynin et al., 1981;Kretschmer et al., 2015;Kvenvolden, 1988;Milkov, 2004). 27

Furthermore, there is a concern that ongoing global warming could result in the dissociation 28

of ~2% of the existing global sub-sea hydrates over the next 800 years (Hunter et al., 2013). 29

However, the fate of the released methane and its impact on the atmosphere and climate 30

remains very uncertain and a quantitative assessment is therefore deemed important for 31

climate projections. 32

In particular, the Arctic environment is a very climatically sensitive region, which is warming 33

fast and twice as much than the rest of the world. This phenomenon is called the Arctic 34

amplification (Screen and Simmonds, 2010). Under the RCP8.5 “business as usual” scenario 35

(Stocker et al., 2013), the Arctic temperature could rise as much as 10-12 degrees by 2100 in 36

certain areas. The Arctic Ocean hosts numerous methane seeps that are presently very active 37

and gas hydrate accumulations that are widespread but with a patchy distribution (Bünz et al., 38

2012;Paull et al., 2007;Ruppel, 2015;Phrampus et al., 2014;Shakhova et al., 2010;Sahling et 39

al., 2014). Some of the methane seeps are directly connected with dissociating methane 40

hydrates (Westbrook et al., 2009;Berndt et al., 2014;Portnov et al., 2016) and/or thawing 41

subsea permafrost (Portnov et al., 2013) after ice sheet retreats. Although the suggested gas 42

hydrate storage in the Arctic represents only a fraction of the global repository of methane 43

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stored in hydrates (Kretschmer et al., 2015;Marín-Moreno et al., 2016), the rapid loss of sea 44

ice (Boe et al., 2009), thawing subsea permafrost, and a warming ocean (Ferré et al., 2012) 45

could potentially result in accelerated release of methane from dissociating hydrates. This 46

study presents a quantitative analysis of the impact of methane hydrate dissociation on the 47

atmosphere and climate over the 21st century. To this end, we analyze methane hydrate 48

dissociation in the Arctic Ocean and potential methane emissions to the atmosphere due to 49

ocean warming. The approach utilizes an ensemble of nine Coupled Model Intercomparison 50

Project (CMIP5) (Taylor et al., 2012) climate predictions to model the transient evolution of 51

hydrate stability with variations in ocean bottom water temperature. The impact of methane 52

emissions resulting from gas hydrate dissociation on the atmosphere is then analyzed through 53

atmospheric chemistry and transport modeling. Finally, we calculate the radiative forcing of 54

the atmospheric perturbation resulting from the release of methane from hydrate dissociation 55

over the next 100 years. 56

2. Methods 57

2.1 Quantification of the present-day methane hydrate reservoir in the Arctic 58

The amount of methane trapped as gas hydrates is calculated based on the thickness of 59

methane hydrate stability zone, hydrate saturation in the sediments, and the porosity. Methane 60

hydrate stability thickness in the Arctic sediments north of 65o N is estimated using the 61

CSMHYD program (Sloan and Koh, 2008) for pure methane gas and a pore-water salinity of 62

35 ‰. The input data for the hydrate stability modeling include the IBCAO bathymetry 63

(Jakobsson et al., 2012) and thermal gradient measurements over the Arctic (Bugge et al., 64

2002;Phrampus et al., 2014;Damm et al., 2013;Pollack et al., 1993) (See S1 for more details). 65

Estimation of hydrate saturation in the sediment pore space involves direct methods such as 66

drilling or indirect methods which involve analysis of seismic data. As there are no direct 67

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measurements of hydrate saturation in the Arctic Ocean, we adopt hydrate saturation 68

estimates derived from analysis of ocean-bottom seismic data from offshore Svalbard (Hustoft 69

et al., 2009;Chabert et al., 2011;Westbrook et al., 2008). Based on these studies, we apply a 70

constant hydrate saturation of 9 ± 3 % of pore space throughout the gas hydrate stability zone 71

in the Arctic sediments. We assume a constant hydrate-free sulfate reduction zone extending 72

from the seafloor to a depth of 5 m in the sediments (Riedel et al., 2006), where anaerobic 73

oxidation consumes methane. The gas hydrate stability zone is adjusted based on the global 74

sediment thickness map in areas where the base of hydrate stability zone exceed the thickness 75

of sediments (Laske and Masters, 1997). We employ a porosity curve, with values from 76

available ocean drilling data (for locations of the drill holes, see Fig. S3) (IODP database, 77

http://iodp.tamu.edu/janusweb/links/links_all.shtml) for the estimation of available pore-space 78

for hydrate formation. To estimate the volume of methane trapped in hydrates, we consider 79

163 m3 of methane trapped in 1 m3 of hydrates with a 94 % cage occupancy based on 80

observations from Blake Ridge (Lorenson and Collett, 2000). The number of moles of 81

methane estimated stems from the methane volume using the ideal gas law at STP. 82

2.2 Transient evolution of methane hydrate reservoir 83

To estimate the evolution of methane hydrate stability in the Arctic during the 21st century 84

(2006-2100), we consider seafloor temperature variations from nine different CMIP5 climate 85

models (Arora et al., 2011;von Salzen et al., 2013;Gent et al., 2011;Hurrell et al., 2013;Dunne 86

et al., 2012;Dunne et al., 2013;Schmidt et al., 2006;Schmidt et al., 2014;Collins et al., 87

2011;Martin et al., 2011;Dufresne et al., 2013;Watanabe et al., 2011;Jungclaus et al., 88

2013;Stevens et al., 2013) (Table S1) based on the RCP 8.5 scenario (Riahi et al., 2011). 89

Using an ensemble of models illustrates the uncertainty range in model simulations and the 90

multi-model mean generally agrees more favorably with observations than the individual 91

models (Flato et al., 2013). It also increases the robustness of results and estimates for the 21st 92

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century. The climate models have a temporal resolution of one year which allows analysis of 93

hydrate stability evolution at a high temporal resolution. A 3D finite-difference heat flow 94

model is used to estimate the diffusive transport of seafloor temperature variation through the 95

sediments (Turcotte and Schubert, 2002;Spiegelman, 2004;Gerya, 2010;Phrampus and 96

Hornbach, 2012) (see S4 for more details). Afterwards, the resulting subsurface temperature 97

profile for each year allows estimating the thickness of the methane hydrate stability zone and 98

the methane stored within the zone as hydrates. 99

2.3 Radiative forcing of released methane from methane hydrates 100

The estimated yearly average methane emissions to the atmosphere from methane hydrate 101

dissociation as predicted by the transient hydrate model were added as an additional emission 102

source in a simulation using the Oslo CTM3 model (Dalsøren et al., 2016;Søvde et al., 2012). 103

The lifetime of methane is longer than the time-scale for interhemispheric exchange, it is 104

therefore not critical for the climate impact study to know exactly where the extra Arctic 105

Ocean gas emissions occur. Emissions are therefore distributed evenly over the ocean north of 106

70° N. The initialization of the atmosphere started with the year 2006 concentrations of 107

methane and other chemical compounds affecting the atmospheric chemistry as described in 108

Dalsøren et al. (2016). See also Dalsøren et al. (2016) for further details on model setup, 109

chemical reactions, and applied meteorological fields. The model was then run with the extra 110

methane flux until the atmospheric methane burden reached a new equilibrium. The 111

calculated change in tropospheric methane concentration was used to quantify the radiative 112

forcing using simplified equations for methane (Myhre et al., 1998). 113

114

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3. Results and Discussion 117

3.1 Methane hydrates in the Arctic and its response to a warming ocean 118

The present-day hydrate stability zone in the Arctic modeled using present-day pressure-119

temperature conditions is shown in figure 1. On the continental shelves off northern Norway, 120

Svalbard, Russia, and Alaska, the pressure-temperature conditions are not suitable for the 121

widespread occurrence of methane hydrate accumulations, except for relatively smaller areas 122

in the Barents Sea. Gas hydrates could be stable under the submarine permafrost off the coast 123

of Russia and Alaska. Permafrost conditions are not considered in this model, however, the 124

hydrate accumulations are relatively small in the marine permafrost regions (Ruppel, 2015). 125

Even today, the depth of the hydrate stability zone may reach a substantial 700 m thickness 126

beneath the seafloor (dark blue) as seen in the Canada Basin, where water depths reach 127

approx. 4000 m (Fig.1). 128

Based on the thickness of the modeled gas hydrate stability zone and hydrate saturation within 129

the sediments we estimate a methane hydrate volume of 29 ± 12 (1-σ) trillion m3, which is 130

equivalent to 4777 ± 1901 (1-σ) trillion m3 of methane or 2524 ± 1005 (1-σ) Gt of carbon. 131

The model does not include hydrates trapped under submarine permafrost on Arctic 132

continental shelves, which may amount to approx. 20 Gt of carbon (Ruppel, 2015). Our 133

estimate falls within the range of values reported previously, with its lowest at 110 Gt of 134

carbon (Kretschmer et al., 2015) and highest at 9000 Gt of carbon (Biastoch et al., 2011). 135

The impact of non-linear variations in the ocean bottom temperature on methane hydrate 136

stability during the 21st century is evaluated by employing an ensemble of nine CMIP5 137

climate models (from 2006-2100) under the RCP8.5 scenario (Fig.2a). The model data were 138

compared to measured bottom water temperatures for the period 1960-2013 at three different 139

locations (Fig.2a). More than half of the selected climate models agree well with the observed 140

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bottom water temperature, except in the Beaufort Shelf, but there the number of 141

measurements is relatively low. The values predicted by the model are yearly averages hence 142

the mismatch between seasonal measurements and model predictions are expected. The 143

climate models reveal bottom water temperatures variations up to 11 0C (e.g., HadGEM2-ES) 144

in the Barents Sea over the 94 years from 2006-2100 (Fig.2a). The lowest variation in bottom 145

water temperature over this period (an increase of <1 0C) is projected by the GISS-E2-R 146

model. 147

In the model, the thickness of the hydrate stability zone varies in response to the changes in 148

bottom water temperature. Due to the non-linearity of bottom water temperature variations, 149

shallow hydrates may form or dissociate if the bottom water gets colder or warmer, 150

respectively. However, since the bottom water temperature increases overall over the 151

investigated period, the total volume of methane hydrates in the Arctic is lower in the year 152

2100 than 2006. We estimate ~0.7 % ± 0.25 (1-σ) of the initial hydrate volume could 153

dissociate until 2100 (Fig. 2b). This corresponds to 32.48 ± 11.6 (1-σ) trillion m3 (22884 ± 154

8173 Tg) of methane or 17.15 ± 6.13 (1-σ) Gt of carbon that could be released into the water 155

column of the Arctic over the course of 94 years, until the year 2100. The amount of methane 156

dissociated from hydrates varies from year to year with a maximum of 428 ± 217 Tg yr-1 of 157

methane during the year 2081 (Fig.3). These variations are mainly due to the fluctuations in 158

ocean bottom temperatures (Fig.2a) resulting in a rapid response of the shallow hydrate 159

system, typically 1-50 m below the seafloor. 160

Our calculations show a mean value of 220 ± 160 Tg yr-1of methane released from hydrate 161

dissociation at the Arctic seabed to the ocean. In our estimate, we assume no heat changes 162

during hydrate dissociation or gas retention in sediments, and no delay in the time taken for 163

the gas to migrate through the sediments to the seafloor. These effects may slow-down 164

methane flux to the water column in the short term (100 years) by up to >70% (Stranne et al., 165

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2016b), hence we present an upper estimate. Biastoch et al. (2011) predicted a mean emission 166

of 162 Tg yr-1 of methane release from the Arctic seabed. A more recent study using a single 167

climate model predicted a reduction of up to 0.12 % of the existing gas hydrate reservoir by 168

the end of the century (Kretschmer et al., 2015). Both these results are of the same order of 169

magnitude as our estimations. 170

The distribution of the methane releases from methane hydrates to the ocean until 2100 are 171

not uniform in the Arctic (Fig.4a). The Arctic continental slopes are hotspots for hydrate 172

dissociation due to ocean warming. The area affected most is the SW Barents Sea, where up 173

to 16000 moles m-2 of methane (0.25 tons m-2) (or a lower limit of 4800 moles m-2, based on 174

Stranne et al. (2016b)) could be released into the water column before 2100 (Fig.4a). Most of 175

the hydrate dissociation occurs in the 350-450 m seafloor depth range. Reported methane 176

seeps from Arctic regions (Fig.4a, red dots), generally match predicted locations of hydrate 177

dissociation, except in permafrost areas which are not considered in our model. From very 178

active methane seep areas on the West Svalbard margin, a methane bubble flow of 4-50 x 106 179

mol yr-1 is reported at a water depth of around 380-390 m using a bubble catcher during the 180

year 2012 (Sahling et al., 2014). This is at least an order of magnitude lower than our mean 181

estimate of 384 x 106 mol yr-1. Modeling conducted over the same area (Marín-Moreno et al., 182

2015) estimate a methane flow from the seabed (420-450 m water depth) of about 25–35 mol 183

yr-1 m-2 from dissociating hydrates. It is comparable to our mean estimate of about 27 mol yr-1 184

m-2 at that location. These estimates are close to methane bubble emissions from thawing sub-185

sea permafrost in the East Siberian Shelf (Shakhova et al., 2014). 186

187

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3.2 Methane release from the Arctic Ocean and its potential impact on the atmosphere and 189

climate 190

The migration of methane from the seabed to the surface water can be either through bubbles 191

or as dissolved methane. Modeling studies suggest that for water depths more than 100 m, 192

methane gas bubbles may not be able to reach the surface water (McGinnis et al., 2006). In 193

addition, methane gas can diffuse out of bubbles, so that most of the methane gas dissolves 194

within the water column before reaching the surface (McGinnis et al., 2006). Various 195

microbial processes and oceanic conditions control the fate of dissolved methane in the water 196

column (AMAPAssessment, 2015). Aerobic microbial oxidation can consume dissolved 197

methane in the water column, the rate of which depends on the amount of methane available 198

and hydrodynamic conditions (Valentine et al., 2001). In addition, water mass stratification is 199

a potential barrier for upward migration of dissolved methane (Graves et al., 2015;Geprägs et 200

al., 2016;Steinle et al., 2015). Recent studies from Western Svalbard identified efficient 201

methane filtering by oxidation and water mass stratification in the water column (Graves et 202

al., 2015;Steinle et al., 2015), and very little to no methane flux into the atmosphere from 203

waters offshore western Svalbard during the 2015 summer season despite high concentrations 204

of methane above the seabed (Myhre et al., 2016). 205

Nevertheless, methane released at seabed could still reach the surface waters under favorable 206

oceanographic conditions as stormy seas, and winter time, and in shallow seas (< 50m) 207

(Shakhova et al., 2014). Analysis of water column methane concentration measurements from 208

the Arctic (Table S2) show that the amount of methane that reaches surface waters is a 209

function of the water depth (Fig.4b) (Mau et al., 2015;Shakhova et al., 2010;Schneider von 210

Deimling et al., 2011;Damm et al., 2008;Damm et al., 2007;Damm et al., 2005;Myhre et al., 211

2016;Gentz et al., 2014;Lammers et al., 1995;Steinle et al., 2015;Graves et al., 2015). More 212

than 50 % of the methane released at the seabed seem to reach the surface waters when the 213

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water depth is lower than 20 m (Fig. 4b, Table S2). However, the amount of methane that 214

reaches the surface water reduce drastically as the water depth increases. Around the water 215

depth of 300-400 m, where most of the hydrate dissociation areas are located (Fig. 4a), the 216

amount of methane that reached surface water was between 0.1 and 7.5% of the amount 217

released at the seabed (Fig. 4b, Table S2). The ocean surface water-atmosphere methane flux 218

depends greatly on the wind conditions and the equilibrium concentration of methane in 219

seawater (Shakhova et al., 2014;Graves et al., 2015). Offshore west Svalbard, sea water-220

atmosphere methane flux was up to ~50 % of the surface water concentration over two 221

seasons (Graves et al., 2015). Based on this we propose that 1% (0.1 to 10%) of the methane 222

released at the seafloor reaching the atmosphere. This translates to an emission of 2.2 Tg CH4 223

yr-1 (0.2 – 22) to the atmosphere from hydrate dissociation in the Arctic. This is about five 224

times lower than the anthropogenic methane emissions in the Arctic in the year 2005 from 225

fossil fuel industry, agriculture, and wastewater sectors, which is estimated to ~56 Tg yr-1 226

(ranging from 56 – 67 as per three different agencies) of CH4 and predicted to increase up to 227

103 Tg yr-1of CH4 by 2050 (based on GAINS CLE) (AMAPAssessment, 2015). 228

To assess the atmospheric changes and calculate the additional radiative forcing due to 229

methane emissions from hydrates dissociation until 2100, we have added emissions of 2.2 Tg 230

CH4 yr-1 (0.2 – 22) to a detailed global chemistry-transport model (Dalsøren et al., 2016) for 231

calculations of the corresponding changes in the atmospheric CH4 concentration. The 232

simulated global mean increase in atmospheric CH4 concentration over the period 2006-2100 233

is estimated to be 13 ppb (1.3 – 130). We calculate a radiative forcing of 0.005 Wm-2 (0.0005 234

to 0.05) from this change in atmospheric methane abundance (Myhre et al., 1998). A change 235

in atmospheric CH4 also changes ozone and stratospheric water vapor. Based on previous 236

simulations, we quantify our upper limit to be less than 50% of the CH4 radiative forcing 237

(Isaksen et al., 2011). The radiative forcing due to the release of methane from hydrate 238

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dissociation to the atmosphere is therefore estimated to be in an upper range of 0.007 Wm-2 239

(0.0007 to 0.07) up till 2100. To put such a change into perspective, this is less than 0.1% of 240

the total radiative forcing in the RCP8.5 scenario. This conclusion holds even with a 25% 241

increase in the radiative forcing of methane recently estimated, mainly due to the inclusion of 242

shortwave absorption by methane (Etminan et al., 2016). 243

Our study suggests that even under strong global warming (RCP 8.5) projections, Arctic 244

methane hydrate dissociation and fluxes to the atmosphere may have a negligible impact on 245

the global climate and thus on a climate feedback loop in the near future (within this century). 246

The added 13 ppb of methane to the atmosphere from hydrate dissociation is comparable to 247

the annual yearly increase in recent years (The global mean increase from 2014-2015 was 11 248

ppb (WMO, 2016)). To reach a 1-degree additional increase in global temperature, it would 249

require three to four times increase in the methane concentration, i.e., on the order of 800 Tg 250

yr-1 of methane release from the Arctic Ocean to the atmosphere over the course of 21st 251

century (Isaksen et al., 2011;Samset et al., 2016). Based on our study, a maximum amount of 252

only 380 Tg yr-1 of methane might release into the Arctic atmosphere due to hydrate 253

dissociation, considering no ocean filter. 254

The contribution of marine methane seepage to the global methane emission is estimated to be 255

~20 Tg yr-1 (Kvenvolden et al., 2001), with a global contribution of 2-10 Tg yr-1 from hydrates 256

(Ciais et al., 2013). Airborne observations of methane in the Arctic Ocean suggest a methane 257

efflux of 10.2 Tg yr-1(Kort et al., 2012). These values, along with the reported emissions on 258

the East Siberian Arctic Shelf (Shakhova et al., 2010), are within our estimated range of 0.2 - 259

22 Tg yr-1. Based on our climate impact analysis and other recent studies (Stranne et al., 260

2016a;Ruppel and Kessler, 2017), gas hydrate dissociation in the Arctic Ocean appears to be a 261

minor methane source to the atmosphere in the near future. 262

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4. Impact of model uncertainties 263

As with any modeling exercise analyzing a complex system, the modeling presented here 264

also involve large uncertainties. Some of these uncertainties arise from the lack of data 265

coverage, whereas a few are inherent due to the complex nature of the Earth system. The 266

uncertainties that arise from the lack of data include heat flow data and sediment porosity. 267

These parameters have a significant impact on the estimation of gas hydrate volume within 268

the Arctic Ocean sediments as presented in the supplementary material (S2, fig. S5). As such, 269

this manuscript is not an effort to improve on the methodology or the estimate of hydrate 270

volume in the Arctic marine sediments. We utilize the volume of hydrates as a marker to 271

evaluate potential scenarios of hydrate dissociation in the 21st century and try to evaluate its 272

impact on climate. The most significant parameters that have a larger impact are hydrate 273

saturation and transport of methane through the sediments and water column. 274

The nature of hydrate distribution through the hydrate stability zone is almost impossible to 275

estimate or model over large regions such as the Arctic Ocean. Hence, we have considered a 276

constant hydrate saturation throughout the hydrate stability zone. This might inflate the gas 277

hydrate volume within the sediments, and the estimates presented here should be taken as a 278

first-order estimate. However, this does not necessarily affect our conclusion, as even with 279

very high estimates of hydrate volume or methane release, the impact on the atmosphere 280

remains minor. 281

As previously mentioned, our study does not account for methane release from permafrost-282

degradation in the Arctic shelves (e.g., Shakhova et al., 2017;Shakhova et al., 2015). 283

Increasing ocean temperatures, seawater transgression, and seafloor erosion are contributing 284

to rapid degradation of marine permafrost which could potentially release methane into 285

shallow water column (Shakhova et al., 2017). Approximately 20 Gt of carbon (~26 Pg of 286

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CH4) might be trapped as methane under the thawing Arctic shelves (<200 m) (Ruppel, 287

2015). Complete destabilization of permafrost generally takes ~5-7 kyr, based on modeling 288

results (Romanovskii et al., 2005). If we consider a rapid degradation of permafrost within a 289

1 kyr period (e.g., Shakhova et al., 2014), the maximum amount of methane that could be 290

released in to the water column is ~26 Tg yr-1. This is well within the uncertainty limits of 291

our estimated 220 ± 160 Tg yr-1 emission from gas hydrate dissociation. 292

As detailed in sections 3.1 and 3.2, the transport of methane is slow through the sediments 293

and methane is consumed both in the shallow sediments as well as within the water column. 294

Methane oxidation in the water column can add to the CO2 budget of the Ocean. However, 295

additional CO2 generated through this process is at least seven orders of magnitude lower 296

than the CO2 influx into the Ocean (e.g., Takahashi et al., 2009;Mau et al., 2013) and has 297

negligible impact on the earth system model used in this study. This is not accounted in our 298

study, as the additional CO2 generaConsidering that most of the hydrate dissociation occur 299

over water depths of 300-400 m, little to no methane might be reaching the atmosphere, as 300

evidenced offshore Svalbard (Myhre et al., 2016). Thus, our model here might be 301

overestimating the methane flux into the atmosphere, as well as its radiative forcing. 302

However, this also does not affect our conclusions as our estimates show that even if the 303

effect of the water column is neglected, the methane flux to the atmosphere is too low to have 304

a significant impact on global temperatures. 305

5. Conclusions 306

Here we present the climate impact of methane emissions from dissociating gas hydrates over 307

the Arctic Ocean in the 21st century by integrating hydrate stability, seafloor temperature 308

variations, and atmospheric modeling. The mean present-day carbon reservoir within Arctic 309

sediments is estimated to be ~2524 ± 1005 Gt. Transient modeling of hydrate stability shows 310

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that up to 32 ± 12 trillion m3 of methane (17 ± 6 Gt C) could release into the Arctic Ocean in 311

the 21st century as a result of warming ocean waters. However, microbial methane filter 312

processes and oceanic conditions restrict the methane release to the atmosphere to about 1% 313

(0.1 – 10%) of the expelled methane at the seafloor. This amounts to methane emission of 2.2 314

Tg CH4 yr-1 (0.2 – 22) to the atmosphere, which results in a radiative forcing of 0.007 Wm-2 315

(0.0007 to 0.07) Wm-2 up until 2100. This represents less than 0.1 % of the total radiative 316

forcing in the RCP8.5 scenario, suggesting that climate impact of methane release from 317

hydrate dissociation appears to be minor in the Arctic Ocean during the 21st century. 318

Data Availability 319

The climate models are available from the sources mentioned in Table S1. The bottom water 320

temperature data, heat flow data, porosity values are accessible from the websites of NOAA-321

NODC, International Heatflow Commission, and the Ocean Drilling Program respectively. 322

323

Competing interests 324

The authors declare that they have no conflict of interest. 325

326

Acknowledgements 327

This research is funded by the Research Council of Norway through the MOCA (Methane 328

Emissions from the Arctic Ocean to the Atmosphere: Present and Future Climate Effects) 329

project (Grant no.225814) and CAGE – Centre for Arctic Gas Hydrate, Environment, and 330

Climate, through its Centres of Excellence funding scheme (Grant no. 223259). For their roles 331

in producing, coordinating, and making available the CMIP5 model output, we acknowledge 332

the climate modeling groups (Table S1), the World Climate Research Programme's (WCRP) 333

Working Group on Coupled Modeling (WGCM), and the Global Organization for Earth 334

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System Science Portals (GO-ESSP). The publication charges for this article have been funded 335

by a grant from the publication fund of UiT-The Arctic University of Norway. 336

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FIGURE 1 719

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Figure 1. Present-day gas hydrate stability zone over the Arctic region modeled in this study 721

(see supplementary S1 for details). 722

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FIGURE 2 726

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Figure 2. a) Comparison between temperature predictions from 9 CMIP5 models and 728

measurements of bottom water temperature from high-resolution CTD (Conductivity, Depth, 729

Temperature) data shown as black ‘+’ symbols (See S1 and S3 for more details). The 730

measured water depth is indicated in meters. b) Variation in the potential volume of methane 731

hydrates within the hydrate stability zone with the changes in ocean bottom temperatures 732

based on multiple climate models until the year 2100. 733

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FIGURE 3 736

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Figure 3. Amount of methane (Tg) released per year from methane hydrates to the ocean over 738

the whole Arctic for each climate model. The mean represents the average amount of methane 739

released each year as predicted by the nine different modeling scenarios. The shaded area 740

represents the standard deviation (1-sigma) for each year. 741

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FIGURE 4 753

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Figure 4. a) Map showing the regions and mean value of total amount of methane released 755

until 2100. Identified methane seeps in the Arctic are also marked. The light-blue line denotes 756

360 m isobath which we identify as the hotspot for future methane release in the Arctic. b) 757

Percentage of methane observed (blue stars, as a function of observed bottom water 758

concentrations) in surface water of Arctic seep locations (See table S2 in supplementary). 759

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climatic impact of arctic ocean methane hydrate dissociation ......climatic impact of arctic ocean...

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